This invention relates generally to thermoelectric coolers and, in particular, to deep thermoelectric (TE) coolers. As used herein, the term, deep TE cooling, refers to Peltier cooling to temperatures below 100 degrees Kelvin (.degree.K.) (for example, liquid nitrogen (LN.sub.2) temperatures, which are 77.degree. K. or lower).
It is well known in the art of thermoelectrics that the low coefficient of performance (COP) of TE coolers has long prevented TE cooling devices from achieving the region of deep cooling. In turn, low COPs are derived from the fact that even the best modern materials for TE cooling show a thermoelectric "figure of merit", ZT, reflecting desirable material properties for thermoelectric cooling, which does not exceed unity.
That is, the cooling capability of a standard TE couple is limited, due mainly to the Joule heat evolution within the bulk of the couple, and particularly due to that portion of the Joule heat flux which normally reaches the cold junction of the couple. Cooling in TE couples is further limited by the heat flux conducted from the hot junction to the cold junction. Thus, the net cooling Q.sub.o at the cold junction may be expressed as: EQU Q.sub.o =I P.sub.T -1/2I.sup.2 R-K.DELTA.T Eq. (1)
where I is the current,
P.sub.T is the Peltier coefficient, and I P.sub.T represents reversible Peltier cooling; PA1 R is the resistance of the TE couple, and 1/2I.sup.2 R represents the half of the irreversible Joule heating which goes to the cold junction; PA1 K is the thermal conductivity of the TE couple, PA1 .DELTA.T is the temperature difference between the second end and cold junction, and PA1 .DELTA.AT represents the heat conduction to the cold junction.
Research efforts in the art of TE cooling over the past four decades have concentrated on the problem of improving and increasing the ZT of known TE materials, i.e. inventing new and better materials to reduce heat losses. However, the increase in ZT required to achieve deep cooling by conventional means is so significant that such a material is unlikely to be developed in the near future. The lowest temperature reached with a TE refrigerator has been at best 134.degree. K., using eight stages in a multi-stage design, and under laboratory conditions. To date, no practical working models of deep TE coolers exist.
Meanwhile, the need exists for deep TE coolers to satisfy the needs for solid state cooling without cryogenic fluids in a broad range of applications, including night vision, advanced electronics, computers, high temperature superconductors, and other applications at cryogenic temperatures.